macOS Monterey
Big Business supports MacOS Monterey with BigBusiness 15 which is certified for it.
Apple has discontinued Monterey but it is the best choice for old machines that will not take a higher operating system but that you would still like to keep in the loop.
BigBusiness 15 is Universal and runs natively on both your old Intel Macs with i5, i7 and your new Apple Silicon Macs with M2, M3 and any mix of supported MacOS.